1. Technical Field of the Invention
This invention most generally relates to methods and apparatus for attenuating the visible wake of marine vehicles. More particularly, it relates to suppressing and attenuating components of the displacement wake of surface vessels and submarines in order to reduce the signature present in such wakes, as it may be observed by passive means from overhead.
2. Background of the Invention
When any solid object or vessel travels through or on the surface of a liquid it creates a disturbance as mechanical energy is transferred to the liquid by force and friction. The disturbance is called a wake and is comprised of two primary mechanical types, a displacement wake and a boundary layer wake. Displacement wakes are caused by the incompressible liquid being pushed out of the way at the bow of the vessel, called a compression wave, and falling back in at the stem, an expansion wave, as it travels through or on the liquid. A displacement wake created on or near the surface takes the physical form of a water wave. A displacement wake travels away from the vessel for great distances, until the mechanical potential energy stored in the waveform is dissipated by the friction in the liquid and in the case of a boundary surface or gas/liquid interface, the friction of the gas in combination with the liquid. In summary, the disturbance is called a wake and the physical form of the displacement wake is a water wave.
In the case of a submarine the displacement wake travels from the submerged vessel to the boundary surface and makes itself evident there just like the disturbance a submerged rock leaves in a swiftly traveling stream. The wake travels away from the point of disturbance until the energy in the wake is dissipated by the medium. This wave component of a vessel's wake can be quite visible from aircraft and space, and can readily lead the observer to determine the location and direction of travel of the vessel that produces the wake. Approximate speed and depth can also determined.
A boundary layer or vorticity wake is caused when an object traveling through a liquid imparts a velocity to the liquid. The liquid moves in the same direction as the object, but ends up spinning in a vortex because of the different velocity profile imparted to the liquid. A boundary layer wake is not a wave but represents kinetic energy transferred to the liquid by giving the liquid a velocity as a result of the passage of the object. The boundary layer wake stays in place as a ship travels away from a reference point until the velocity imparted to the liquid diffuses. The boundary layer wake of a submarine will not diffuse to the surface unless the submarine is very close to the surface. Furey's U.S. Pat. No. 5,222,455 Ship Wake Vorticity Suppressor, Jun. 29, 1993, covers a method of minimizing the boundary layer wake, but does not address displacement wakes.
A propeller driving a ship or submarine also contributes to the wake created by the vessel. The rotation of the propeller in the liquid imparts a rotational velocity to the liquid. The propeller pushing the vessel through the liquid creates a compression and expansion wave. The rotational velocity imparted to the liquid by the propeller is very much like the boundary layer wake caused by the hull's surface as it travels through the liquid. The displacement wake from the propeller is reversed from the displacement wake created by the vessel. The expansion wake occurs on the forward side of the propeller. The compression wake forms on the rear side of the propeller as the propeller pushes the liquid to propel the vessel.
The energy in a wave created by the passage of a vessel can be attenuated by reflection of the wave to cause interference and scattering of the wave. A second method to attenuate the wave is passing the wave through wave guides that are sized or configured in relation to the wave size to attenuate or dissipate the wave rather than pass the wave efficiently as a wave guide would. The dissipation of the displacement wake is important because ship or submarine can be seen by normal vision, SER (Surface Effect Radar) and SAR (Synthetic Aperture Radar) of the opposing forces. Since the size of the vessel's wake is dependent on the speed the vessel is traveling, suppression or reduction by other means of a ship's or submarine's displacement wake allows a vessel to travel faster without exceeding the threshold level of disturbance that may be detectable by opposing forces.
Referring to FIG. 1, this diagram shows a top view of hull 10 of a typical surface vessel in forward motion through a liquid medium. Normal compression wave 11 and expansion wave 12 are being generated off the bow and stern respectively. Vorticity wake 13 trails off the stem. The three different wakes are clearly visible from above.
Referring to FIG. 2, incident wave 21 is directed towards surface 20 at angle 22 from normal, and results in reflected wave 23 being transmitted from surface 20 at angle 24 from normal. Angle 24 is equal to angle 22.
Referring to FIG. 3, wave 31 is shown to be summed with in-phase wave 32 of equal amplitude to produce wave 33 of double the amplitude. Wave 34 is 180 degree out of phase with wave 35, and when summed results in the canceling out of all amplitude.
Referring to FIG. 4, wave 41 is shown to be transmitted through wave guide 40 without any loss of amplitude, while wave 42 with a different frequency or wave length is unable to be transmitted through the same size wave guide with out a loss of amplitude. A wave guide will pass or channel a wave if the size of the wave guide is appropriate for the wave that is applied. If the wave guide is improperly sized for the wave then the wave will not pass through without a loss of aptitude or amplitude.
The prior art reveals vessels equipped with structures outboard of the hull, but previous patents use the structure to protect the ships from torpedoes and other threats to the ship. In two patents that claim to reduce the displacement wake of the ship, the process by which the displacement is reduced relates to affecting the laminar flow of water past the hull of the vessel.
Russell's U.S. Pat. No. 1,377,222, May 10, 1921, illustrates a structure external to the hull which would interfere with the otherwise normal distribution of wake, but which is intended to perform as a buffer for protection from impact or collision with terrain or external objects. There are two distinguishing features on the ship shown in the figures of the patent: item 2 the chambered buffer and item 13 the shields. The shields on the forward half of the ship would interfere with the bow wave, but the orientation of the rear shields would allow the wake to pass by with little interference. The boat's attachment shields are allowed to travel up and down without consideration of the wake.
Peterson's U.S. Pat. No. 2,356,691 Antitorpedo Shield, Aug. 22, 1944, discloses shields suspended external to the hull to block torpedoes. This specification does not anticipate or address the instant problem.
Hambrick's U.S. Pat. No. 2,475,239 Ship Protector, Jul. 5, 1949, discloses another variation on structures supported external of the hull for collision protection. This design would increase the wake turbulence of the ship.
Osawa's U.S. Pat. No. 5,088,433 Wave-Making Resistance Suppressing means in Ship, Feb. 18, 1992, reveals fins or deflectors attached to the forward end of the hull at the waterline, hinged at their front edge and adjustable to be angled outward at up to 25 degrees to affect the flow of water around the hull. This design would appear to increase the size of the displacement wake for vessels with any appreciable forward speed. Also, it does not affect the stern or propellor wakes.
Petromanolakis's U.S. Pat. No. 5,566,634 Ship's Stem Duct with Airfoil Section, Oct. 22, 1996, claims reduction of the propulsion wave height of the vessel by means of an airfoil shaped bow-mounted duct structure that imparts a further parting force to the water as the bow progresses. This disclosure does not address the stern displacement wake of the vessel or suppressing a major portion of the ship's total displacement wake. There is likewise no suggestion of application to submarines.